The research described in this proposal is concerned with the interrelationships between specific lipids governing their relative concentrations in a biological membrane and selected aspects of membrane lipid synthesis. Using cultured mouse fibroblasts (LM cells) and E. coli K-12, genetic and nutritional methods will be used to change or perturb normal membrane lipid composition in a very specific manner, promoting lipid lateral phase separations and/or secondary adjustments in membrane lipid composition. Biochemical, biophysical and genetic approaches will be employed to characterize the effect of these perturbations on the lipid compositions, physical properties, and functions of the surface membrane. The close relationship between sterol and unsaturated fatty acid synthesis and the mechanism(s) regulating membrane lipid synthesis in response to sterol depletion will be studied. A long term goal for the project is the identification of the alarm signal or primary means by which undesirable perturbations in membrane properties are detected. Control of membrane structure is fundamental to the maintenance of numerous cellular and biological functions.